Liquid crystal display device

ABSTRACT

A doubler part doubles frequencies of video signals. A drive control circuit generates, in response to a synchronizing signal outputted from the doubler part, PWM dimming frequency information such that a PWM dimming frequency f and a black display ratio B satisfy the relationships f≧25B+250 and B&gt;10, and provides such information to a PWM dimming signal generation circuit. In addition, the drive control circuit drives a gate driver and a source driver such that one frame period is divided into an image display period and a black display period. The PWM dimming signal generation circuit generates, in response to a synchronizing signal and the PWM dimming frequency information, a PWM dimming signal and provides the PWM dimming signal to a lighting circuit. The lighting circuit activates a backlight device with dimming, in response to the PWM dimming signal. This configuration reduces colored interference fringes in a liquid crystal display device resulting from the combination of a black insertion drive technique and a PWM dimming technique.

TECHNICAL FIELD

The present invention relates to a liquid crystal display, and moreparticularly to a liquid crystal display device that displays images byirradiating a liquid crystal panel, which is driven in response to videosignals, with light outputted from a backlight.

BACKGROUND ART

Liquid crystal display devices are so-called hold-type image displaydevices in which the signal level is held, as shown in FIG. 17, for oneframe period in each liquid crystal cell. For the types of liquidcrystals used in liquid crystal display devices, conventionally, a TN(Twisted Nematic) mode liquid crystal is commonly used, but in recentyears, in order to overcome the drawbacks of the TN-mode liquid crystal(e.g., a narrow viewing angle and a slow response time), liquid crystaldisplay devices using an OCB (Optically Self-Compensated Birefringence)mode liquid crystal have been studied. Such devices are disclosed, forexample, in Japanese Laid-Open Patent Publication Nos. 7-84254 and9-96790. As is disclosed in Japanese Laid-Open Patent Publication No.9-96790, the OCB mode requires some kind of initialization process inwhich the state of a liquid crystal cell is changed (hereinafterreferred to as a “transition”) from a splay alignment to a bendalignment by application of a high voltage (which would result in ablack display in the case of normally-white). However, after theinitialization process, once the applied voltage to the liquid crystalbecomes less than a predetermined value Va, the state of the liquidcrystal cell returns to the splay alignment (hereinafter referred to asa reverse transition). For this reason, the OCB mode can be used only inan applied voltage range (Va to Vblack) which allows the bend alignmentto be maintained, such as the one shown by the curve a in FIG. 18.

It has been found, however, that even if a period exists in which theapplied voltage to the liquid crystal temporarily becomes less than thepredetermined value Va, if a high voltage is periodically applied inperiods other than the aforementioned period, a reverse transition doesnot occur. For example, in a liquid crystal display device disclosed inJapanese Laid-Open Patent Publication No. 2000-31790, the frequencies ofvideo signals are doubled, each gate line is selected twice in eachframe period, and a video signal and a signal for applying theaforementioned high voltage are written alternately to each pixel of theliquid crystal panel (each signal is written once in one frame period).This makes it possible to use a wider applied voltage range, such as theone shown by the curve b in FIG. 18. It is known that the minimumhigh-voltage application period that ensures elimination of reversetransition (hereinafter referred to as black display period) is a periodwhich is about 10% of one frame period.

Meanwhile, as for improvement in the response time of liquid crystal, ithas been reported that in the TN-mode liquid crystal, by reducing thecell gap from about 5 μm, which is conventionally employed, to about 2μm, the response time of a liquid crystal can be made shorter than oneframe period (16.6 ms).

By employing the black insertion drive technique in a liquid crystalpanel with a fast response time, such as a liquid crystal panel usingthe aforementioned OCB-mode liquid crystal or a liquid crystal panelusing a TN-mode liquid crystal in which the cell gap is reduced to about2 μm, the edge blurring when displaying a moving image is expected to begreatly reduced.

As a method of controlling the luminance of a backlight of a liquidcrystal display device, conventionally, a voltage dimming technique anda PWM (Pulse Width) dimming technique are widely employed. The voltagedimming technique controls luminance by changing the applied voltage toa fluorescent lamp, which serves as a backlight source. The PMW dimmingtechnique controls luminance in a manner such that, as shown in FIG. 19,dimming is performed in response to a PWM dimming signal having aperiodic rectangular waveform. The lamp current is allowed to flow onlyduring an ON period (pulse width) of the signal.

The voltage dimming technique, though its circuit configuration issimple, has drawbacks. For example, when the drive voltage is low,proper lighting of the fluorescent lamp is difficult to obtain. On theother hand, in the PWM dimming technique, though the luminance of thefluorescent lamp can be easily controlled, there is a drawback in thatswitching noise occurs at the time of dimming. When controlling thelighting of the backlight by the PWM dimming technique, if the dimmingfrequency is increased, the luminance efficiency is greatly reduced dueto switching losses, etc., and therefore the dimming frequency istypically set to 300 Hz or less.

Meanwhile, it has been confirmed by an observation performed by theinventors that when backlight control by the PWM dimming technique andthe aforementioned black insertion drive technique are simultaneouslyperformed, color non-uniformity, as shown in FIG. 20, such that aproperly displayed portion c and a luminance-reduction portion daccompanied with coloring are displayed alternately, occurs in anentire-screen white display state. The cause of this colornon-uniformity is briefly described below.

The content to be displayed on the liquid crystal display device isdefined by the product of the amount of light emitted from the backlightmultiplied by the transmittance of the liquid crystal panel, and inpractice, the time-average value of this product is perceived by theviewer's eye. In the aforementioned properly displayed portion c in FIG.20, an operation such as that shown in FIG. 21 is performed. That is,the light-off period of the backlight in PWM dimming coincides with theblack display period of the liquid crystal panel, and therefore theactual display content is hardly adversely affected and a reduction inluminance hardly occurs. (In practice, light is emitted even during thelight-off period due to the persistence characteristics of phosphors,and thus a slight reduction in luminance is caused.)

On the other hand, in the colored luminance-reduction portion din FIG.20, an operation such as that shown in FIG. 22 is performed. That is,the light-on period in PWM dimming coincides with the black displayperiod of the liquid crystal panel, and therefore a reduction inluminance is caused in the actual display content. For phosphors whichare generally widely used in liquid crystal display devices, Y₂O₃:Eu³⁺is used as a red-emitting phosphor, LaPO₄:Tb³⁺ is used as agreen-emitting phosphor, and BaMgAl₁₀O₁₇:Eu²⁺ is used as a blue-emittingphosphor. The 1/10 persistence time of the red, green, and blue emittingphosphors are about 3 ms, about 8 ms, and about 0.1 ms or less,respectively. As can be seen, in the persistence components of thebacklight there are great differences in the persistence time betweenthe phosphors, and thus coloring occurs in the coloredluminance-reduction portion d.

Accordingly, an object of the present invention is to reduce coloredinterference fringes in a liquid crystal display device resulting fromthe combination of the black insertion drive technique and the PWMdimming technique.

SUMMARY OF THE INVENTION

To achieve the above object, the present invention has the followingaspect. It is to be understood that reference numerals, etc., inparentheses are provided, for the purpose of helping to understand thepresent invention, to show the corresponding relationship withembodiments, as will be described later, and thus are not intended tolimit the scope of the present invention.

A liquid crystal display device of the present invention displays imagesby irradiating a liquid crystal panel (11), which is driven in responseto video signals, with light outputted from a backlight device (15). Theliquid crystal display device comprises: drive means (10 and 14) fordriving the liquid crystal panel in response to the video signals in amanner such that one frame period is divided into a black display periodand an image display period; a PWM dimming signal generation circuit(17) for generating a PWM dimming signal for controlling the backlightdevice by a PWM dimming technique; a lighting circuit (16) for drivingthe backlight device in response to the PWM dimming signal; and means(18, 28, 34, and 53) for controlling a cycle and/or phase of the PWMdimming signal to prevent occurrence of interference fringes in theliquid crystal panel, caused by the PWM dimming technique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 1 of the present invention.

FIG. 2 is a diagram showing the relationship between the operation of adoubler part and a black display period and an image display period.

FIG. 3 is a diagram showing the operation of Embodiment 1.

FIG. 4 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 2 of the present invention.

FIG. 5 is a diagram showing the operation of Embodiment 2.

FIG. 6 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 3 of the present invention.

FIG. 7 is an illustrative diagram illustrating the principle where thedegree of color non-uniformity changes with PWM dimming frequencies.

FIG. 8 is a diagram showing the relationship between the PWM dimmingfrequency and the color difference in color non-uniformity for differentblack display ratios.

FIG. 9 is a diagram showing conditions that a black display ratio and aPWM dimming frequency should satisfy to prevent occurrence of colornon-uniformity.

FIG. 10 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 4 of the present invention.

FIGS. 11( a) and 11(b) are illustrative diagrams showing therelationship between 1/10 persistence time and color non-uniformity.

FIG. 12 is a diagram showing luminance efficiency versus PWM dimmingfrequency.

FIG. 13 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 5 of the present invention.

FIG. 14 is a diagram showing the operation of Embodiment 5.

FIG. 15 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 6 of the present invention.

FIG. 16 is a diagram showing the operation of Embodiment 6.

FIG. 17 is an illustrative diagram showing a display signal in aconventional liquid crystal display device.

FIG. 18 is an illustrative diagram showing a black insertion drivetechnique in an OCB-mode liquid crystal.

FIG. 19 is an illustrative diagram showing a PWM dimming technique in abacklight.

FIG. 20 is an illustrative diagram showing color non-uniformityresulting from the combination of the black insertion drive techniqueand the PWM dimming technique.

FIG. 21 is a diagram showing the operation in a properly displayedportion in a conventional liquid crystal display device.

FIG. 22 is a diagram showing the operation in a coloredluminance-reduction portion in the conventional liquid crystal displaydevice.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to the drawings, various embodiments of the presentinvention are described below.

(Embodiment 1)

FIG. 1 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 1 of the present invention. Theliquid crystal display device includes a doubler part 10, a liquidcrystal panel 11, a gate driver 12, a source driver 13, a drive controlcircuit 14, a backlight device 15, a lighting circuit 16, a PWM dimmingsignal generation circuit 17, and a control signal generation circuit18.

To the liquid crystal display device are fed video signals and asynchronizing signal. The doubler part 10 doubles the frequencies of thevideo signals in response to the video and synchronizing signals. Then,the doubler part 10 provides a source signal to the source driver 13 andalso provides the frequency-doubled synchronizing signal to the drivecontrol circuit 14, the control signal generation circuit 18, and thePWM dimming signal generation circuit 17. Here, as the source signal, asshown in FIG. 2, original video signals (S1, S2, S3, . . . ) andnon-video signals (B) are outputted alternately. The non-video signalserves to apply a high voltage to the liquid crystal panel 11, andcorresponds to a black display.

The control signal generation circuit 18 receives the synchronizingsignal outputted from the doubler part 10, generates black displayperiod information such that the black display period is an integralmultiple of a PWM dimming cycle and PWM dimming cycle information, andthen provides them to the drive control circuit 14 and the PWM dimmingsignal generation circuit 17, respectively. The drive control circuit 14outputs a clock for driving the source driver 13 and a gate signal fordriving the gate driver 12, in response to the aforementioned blackdisplay period information and frequency-doubled synchronizing signaloutputted from the doubler part 10. The gate driver 12 outputs, inresponse to the gate signal, gate pulses (GP1 to GP8), such as thoseshown in FIG. 2, to gate lines of the liquid crystal panel 11,respectively. For simplicity of description, FIG. 2 shows the case whereeight gate lines are present. To pixels of each gate line of the liquidcrystal panel 11, a non-video signal and a video signal are each writtenonce in one frame period. In the description below, the period of timefrom when a non-video signal is written until a video signal is writtenis referred to as a black display period, and the period of time fromwhen a video signal is written until a non-video signal is written isreferred to as an image display period.

The PWM dimming signal generation circuit 17 generates a PWM dimmingsignal in response to the synchronizing signal and the aforementionedPWM dimming cycle information, and provides the PWM dimming signal tothe lighting circuit 16. The lighting circuit 16 activates the backlightdevice 15 with dimming, in response to the PWM dimming signal.

With reference to FIG. 3, the operation of the present embodiment isdescribed in detail below.

In the present embodiment, by the control signal generation circuit 18,the black display period is set to be an integral multiple (double inthe example in FIG. 3) of the PWM dimming cycle in the backlight, asshown in FIG. 3. Accordingly, light emitted from the backlight device 15is shielded during the black display period for a period equal to anintegral multiple of the PWM dimming cycle. Thus, the time-average valueof the ratio of the amount of backlight light and persistence componentsshielded in the black display period becomes more uniform across theentire screen, thereby reducing non-uniformity of luminance and color.

For the liquid crystal panel 11, it is preferable to use either anOCB-mode liquid crystal panel or an TN-mode liquid crystal panel with acell gap of less than 5 μm (preferably about 2 μm), because in suchpanels the response time of liquid crystal is fast, and accordingly,edge blurring of a moving image can be further reduced.

In the present embodiment, the black display period is set to be anintegral multiple of the PWM dimming cycle. However, needless to say,even if it is not exactly an integral multiple, as long as the blackdisplay period is set to have the relationship close thereto,substantially the same effects are achieved. For example, if the blackdisplay period satisfies the following relationship:Black display period=(integer)·(PWM dimming cycle)±0.3·(PWM dimmingcycle),favorable effects are achieved.

As described above, according to the present embodiment, since the blackdisplay period is set to be an integral multiple of the PWM dimmingcycle, it is possible to reduce colored interference fringes in a liquidcrystal display device resulting from the combination of the blackinsertion drive technique and the PWM dimming technique.

(Embodiment 2)

FIG. 4 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 2 of the present invention. Theliquid crystal display device includes a doubler part 10, a liquidcrystal panel 11, a gate driver 12, a source driver 13, a drive controlcircuit 14, a backlight device 15, a lighting circuit 16, a PWM dimmingsignal generation circuit 17, and a control signal generation circuit28. In FIG. 4, the elements corresponding to those found in FIG. 1 aredesignated by like reference numerals and the descriptions thereof areomitted.

The control signal generation circuit 28 receives a synchronizing signaloutputted from the doubler part 10, generates PWM dimming frequencyinformation such that the PWM dimming frequency is (an odd number/2)times the vertical frequency, and then provides such information to thePWM dimming signal generation circuit 17.

With reference to FIG. 5, the operation of the present embodiment isdescribed in detail below.

In the present embodiment, by the control signal generation circuit 28,the PWM dimming frequency is set to be (an odd number/2) times thevertical frequency, as shown in FIG. 5. This indicates a state in whichthe backlight device 15 is activated in a manner similar to aninterleaved mode. The waveform of the backlight luminance, which hasbeen dimmed by the PWM dimming technique, has a shape such that alighting-delayed part in a light-on period and persistencecharacteristics in a light-off period are substantially inverted withrespect to each other. Thus, by setting the PWM dimming frequency to be(an odd number/2) times the vertical frequency, the light-on period andlight-off period of the backlight alternately correspond, frame byframe, to the black display period. Accordingly, the time-average valueof the ratio of the amount of backlight light and persistence componentsshielded in the black display period becomes more uniform across theentire screen, thereby reducing non-uniformity of luminance and color.

For the liquid crystal panel 11, it is preferable to use either anOCB-mode liquid crystal panel or an TN-mode liquid crystal panel with acell gap of less than 5 μm (preferably about 2 μm), because in suchpanels the response time of liquid crystal is fast, and accordingly,edge blurring of a moving image can be further reduced.

In the present embodiment, the PWM dimming frequency is set to be (anodd number/2) times the vertical frequency but, needless to say, even ifit is not exactly (an odd number/2) times, as long as the PWM dimmingfrequency and the vertical frequency have the relationship closethereto, substantially the same effects are achieved. For example, ifthe PWM dimming frequency satisfies the following relationship:PWM dimming frequency=(an odd number/2)·(verticalfrequency)±0.2·(vertical frequency),favorable effects are achieved.

As is described above, according to the present embodiment, since thePWM dimming frequency is set to be (an odd number/2) times the verticalfrequency, it is possible to reduce colored interference fringes in aliquid crystal display device resulting from the combination of theblack insertion drive technique and the PWM dimming technique.

(Embodiment 3)

FIG. 6 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 3 of the present invention. Theliquid crystal display device includes a doubler part 10, a liquidcrystal panel 11, a gate driver 12, a source driver 13, a drive controlcircuit 34, a backlight device 15, a lighting circuit 16, and a PWMdimming signal generation circuit 17. In FIG. 6, the elementscorresponding to those found in FIG. 1 are designated by like referencenumerals and the descriptions thereof are omitted.

The drive control circuit 34 generates, in response to a synchronizingsignal outputted from the doubler part 10, PWM dimming frequencyinformation such that a PWM dimming frequency f and a black displayratio B satisfy the relationships f ≧25B+250 and B>10, and provides suchinformation PWM dimming signal generation circuit 17.

With reference to FIG. 7, the principle of the present embodiment isdescribed below.

The relationship between the PWM dimming frequency and the degree ofcoloring is, as shown in FIG. 7, such that the lower the PWM dimmingfrequency, the higher the degree of coloring. The present inventors haveexamined the relationship between the PWM dimming frequency of thebacklight and the color difference in chromaticity variation in a liquidcrystal display device using an OCB-mode liquid crystal. FIG. 8 is adiagram showing the relationship between the PWM dimming frequency andthe color difference in color non-uniformity ΔEuv* (color difference inCIE 1976 L*u*v* color space) for various ratios of black display periodto one frame period (hereinafter referred to as black display ratios).It can be seen that there is a tendency that as the black display ratiodecreases, the color difference ΔEuv* decreases, and as the PWM dimmingfrequency increases, the color difference ΔEuv* decreases. The minimumcolor difference that the human can perceive is generally said to beΔEuv* =1 (see, for example, Noboru Ohta, “Basics of Color ReproductionOptics (Iro Saigen Kogaku No Kiso),” Corona Publishing Co., Ltd., p.46).When the PWM dimming frequency f (Hz), at which the color differenceΔEuv* =1, is plotted from data of each of the black display ratios B(%), the plotted values are distributed around the line f=25B+250, asshown in FIG. 9. If the borderline where color non-uniformity occurs isdefined by the line f=25B+250, the region where no color non-uniformityoccurs is such a region that satisfies the condition f25B+250. It is tobe noted, however, that as is described above, in the case of performinga black insertion drive in an OCB-mode liquid crystal, the black displayratio B (%) needs to be such that B>10. Otherwise, a reverse transitionoccurs and normal functions are impaired, and therefore, the regionwhere no color non-uniformity occurs in an OCB-mode liquid crystal issuch a region that satisfies the conditions f25B+250 and B>10, such asthe shaded area in FIG. 9.

For the liquid crystal panel 11, it is preferable to use either anOCB-mode liquid crystal panel or an TN-mode liquid crystal panel with acell gap of less than 5 μm (preferably about 2 μm), because in suchpanels the response time of liquid crystal is fast, and accordingly,edge blurring of a moving image can be further reduced.

In the present embodiment, the PWM dimming frequency f and the blackdisplay ratio B are set to satisfy the relationships f ≧25B+250 andB>10, but the condition B>10 is specific to an OCB-mode liquid crystaland thus is not essential in the case of using other liquid crystals.

As is described above, according to the present embodiment, since thePWM dimming frequency f and the black display ratio B are set to satisfythe relationship f ≧25B+250, it is possible to reduce non-uniformity ofluminance and color in a liquid crystal display device resulting fromthe combination of the black insertion drive technique and the PWMdimming technique.

(Embodiment 4)

FIG. 10 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 4 of the present invention. Theliquid crystal display device includes a liquid crystal panel 11, adoubler part 10, a gate driver 12, a source driver 13, a drive controlcircuit 14, a backlight device 45, a lighting circuit 16, and a PWMdimming signal generation circuit 17. In FIG. 10, the elementscorresponding to those found in FIG. 1 are designated by like referencenumerals and the descriptions thereof are omitted.

In a fluorescent lamp of the backlight device 45, phosphors are used inwhich the 1/10 persistence time is 40 ms or greater.

The drive control circuit 14 drives, in response to a synchronizingsignal outputted from the doubler part 10, the gate driver 12 and thesource driver 13 in a manner such that one frame period is divided intoan image display period and a black display period. The PWM dimmingsignal generation circuit 17 provides a PWM dimming signal to thelighting circuit 16. The lighting circuit 16 activates the backlightdevice 45 with dimming, in response to the PWM dimming signal.

In the present invention, in the fluorescent lamp of the backlightdevice 45, phosphors are used in which the 1/10 persistence time is 40ms or greater. With reference to FIGS. 11( a) and 11(b), the effectsthereof are described below. FIGS. 11( a) and 11(b) are diagrams showingthe persistence components of the backlight device in a light-off periodfor various phosphors. FIG. 11( a) shows the case of using phosphorswhich are typically used in liquid crystal display devices and have a1/10 persistence time of about 8 ms, and FIG. 11( b) shows the case ofusing phosphors having a 1/10 persistence time of 40 ms or greater. Asis clear by comparing FIGS. 11( a) and 11(b), in the case of usingphosphors having a 1/10 persistence time of 40 ms or greater, thepersistence time of the backlight is sufficiently long compared to thePWM dimming cycle and thus the off-balance of the persistence componentsbetween RGB is small. Accordingly, non-uniformity of luminance and colorcan be reduced.

For the liquid crystal panel 11, it is preferable to use either anOCB-mode liquid crystal panel or an TN-mode liquid crystal panel with acell gap of less than 5 μm (preferably about 2 μm), because in suchpanels the response time of liquid crystal is fast, and accordingly,edge blurring of a moving image can be further reduced.

In the present embodiment, in a fluorescent lamp of the backlight device45, phosphors are used in which the 1/10 persistence time is 40 ms orgreater, but needless to say, even with phosphors in which the 1/10persistence time is close to 40 ms, substantially the same effects areachieved.

As is described above, according to the present embodiment, sincephosphors in which the 1/10 persistence time is 40 ms or greater areused in a fluorescent lamp of the backlight device 45, it is possible toreduce colored interference fringes in a liquid crystal display deviceresulting from the combination of the black insertion drive techniqueand the PWM dimming technique.

(Embodiment 5)

In the foregoing Embodiment 3, color fringes are reduced through drivingwith a PWM dimming frequency which is sufficiently high compared to thatof conventional ones, by relying on the black insertion ratio. However,when the PWM dimming frequency is increased, switching loss tends tooccur more frequently, which in turn reduces luminance efficiency, asshown in FIG. 12. In Embodiment 5, a liquid crystal display device isdescribed with which color fringes can be reduced without the need toincrease the PWM dimming frequency.

FIG. 13 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 5 of the present invention. Theliquid crystal display device includes a liquid crystal panel 11, a gatedriver 12, a source driver 13, a drive control circuit 14, a PWM dimmingsignal generation circuit 17, a control signal generation circuit 18, adirect-type backlight device 50, a first delay circuit 53, a firstlighting circuit 51, and a second lighting circuit 52. The direct-typebacklight device 50 includes a plurality of fluorescent lamps L1 to L8.In FIG. 13, the elements corresponding to those found in FIG. 1 aredesignated by like reference numerals and the descriptions thereof areomitted.

The PWM dimming signal generation circuit 17 generates a first PWMdimming signal. The first delay circuit 53 receives this first PWMdimming signal and generates a second PWM dimming signal such that thePWM dimming phase of the first PWM dimming signal is shifted byapproximately 180°. The first and second PWM dimming signals areprovided to the first lighting circuit 51 and the second lightingcircuit 52, respectively. Meanwhile, fluorescent lamps of an order i,which satisfies the relationship (2n−2)M+1≦i≦(2n−1)M, are all activatedwith dimming, by the first lighting circuit 51 in response to the firstPWM dimming signal, and fluorescent lamps of an order j, which satisfiesthe relationship (2n−1)M+1≦j≦2nM, are all activated with dimming, by thesecond lighting circuit 52 in response to the second PWM dimming signal,wherein i and j (i, j=1, 2, 3, . . . ) are natural numbers thatrepresent the order of the fluorescent lamps starting from one end ofthe backlight in the direct-type backlight device 50, n (n=1, 2, 3, . .. ) is an arbitrary natural number, and M (M=1, 2, 3, . . . ) is anarbitrary natural number. With such an arrangement, lights emitted fromthe fluorescent lamps, which are activated in response to the first andsecond PWM dimming signals, are easily spatially averaged when projectedonto the liquid crystal panel 11. The present embodiment describes thecase where n=1, 2 and M=2.

With reference to FIG. 14, the operation of the present embodiment isdescribed in detail below.

In the present embodiment, by two types of PWM dimming signals, as shownin FIG. 14, i.e., the first PWM dimming signal generated by the PWMdimming signal generation circuit 17 and the second PWM dimming signalsuch that the first PWM dimming signal is delayed by 180° by the firstdelay circuit 53 comprising, for example, a shift resistor, sets offluorescent lamps made from the eight fluorescent lamps L1 to L8 (a setconsisting of L1, L2, L5, and L6 and a set consisting of L3, L4, L7, andL8) are alternately operated with dimming. Consequently, light emittedfrom the backlight device 50 is operated as if the PWM dimming frequencywere spatial averagely doubled. Accordingly, non-uniformity of luminanceand color can be reduced to the same degree as conventional ones, at aPWM dimming frequency which is about half of that conventionallyrequired, and lighting efficiency is improved.

For the liquid crystal panel 11, it is preferable to use either anOCB-mode liquid crystal panel or a TN-mode liquid crystal panel with acell gap of less than 5 μm (preferably about 2 μm), because in suchpanels the response time of liquid crystal is fast, and accordingly,edge blurring of a moving image can be further reduced.

In the present embodiment, the phase difference between the first andsecond PWM dimming signals is set to 180° but, needless to say, even ifthe phase difference is not exactly 180°, as long as it is close to180°, substantially the same effects are achieved.

As described above, according to the present embodiment, because lightsemitted from the fluorescent lamps L1 to L8, which are activated inresponse to the first and second PWM dimming signals, are spatiallyaveraged on the liquid crystal panel and the apparent PWM dimmingfrequency is doubled, colored interference fringes in a liquid crystaldisplay device resulting from the combination of the black insertiondrive technique and the PWM dimming technique can be reduced to the samedegree as conventional ones, at a PWM dimming frequency which is half ofthat conventionally required, and lighting efficiency can be improvedcompared to conventional ones.

(Embodiment 6)

FIG. 15 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 6 of the present invention. Theliquid crystal display device includes a liquid crystal panel 11, a gatedriver 12, a source driver 13, a drive control circuit 14, a PWM dimmingsignal generation circuit 17, a control signal generation circuit 18, adirect-type backlight device 50, a first delay circuit 53, a seconddelay circuit 55, a first lighting circuit 51, a second lighting circuit52, and a third lighting circuit 54. The direct-type backlight device 50includes a plurality of fluorescent lamps L1 to L9. In FIG. 15, theelements corresponding to those found in FIG. 13 are designated by likereference numerals and the descriptions thereof are omitted.

The PWM dimming signal generation circuit 17 generates a first PWMdimming signal. The first delay circuit 53 receives this first PWMdimming signal and generates a second PWM dimming signal such that thePWM dimming phase of the first PWM dimming signal is delayed by about120°, and the second delay circuit 55 receives this second PWM dimmingsignal and generates a third PWM dimming signal such that the phase ofthe second PWM dimming signal is delayed by about 120°. The first,second, and third PWM dimming signals are provided to the first, second,and third lighting circuits 51, 52, and 54, respectively. Meanwhile,fluorescent lamps of an order i′, which satisfies the relationship(3n′−3)M′+1≦i′≦(3n′−2)M′, are all activated with dimming, by the firstlighting circuit 51 in response to the first PWM dimming signal,fluorescent lamps of an order j′, which satisfies the relationship(3n′−2)M′+1≦j′≦2(3n′−1)M′, are all activated with dimming, by the secondlighting circuit 52 in response to the second PWM dimming signal, andfluorescent lamps of an order k′, which satisfies the relationship(3n′−1)M′+1≦k′≦3n′ M′, are all activated with dimming, by the thirdlighting circuit 54 in response to the third PWM dimming signal, whereini′, j′, and k′ (i′, j′, k′=1, 2, 3, . . . ) are natural numbers thatrepresent the order of the fluorescent lamps starting from one end ofthe backlight of the direct-type backlight device, n′ (n′=1, 2, 3, . . .) is an arbitrary natural number, and M′ (M′=1, 2, 3, . . . ) is anarbitrary natural number. With such an arrangement, lights emitted fromthe fluorescent lamps, which are activated in response to the first,second, and third PWM dimming signals, are easily spatially averagedwhen projected onto the liquid crystal panel 11. The present embodimentdescribes the case where n′=1, 2, 3 and M′=1.

With reference to FIG. 16, the operation of the present embodiment isdescribed in detail below.

In the present embodiment, by three types of PWM dimming signals, asshown in FIG. 16, i.e., the first PWM dimming signal generated by thePWM dimming signal generation circuit 17, the second PWM dimming signalsuch that the first PWM dimming signal is delayed by 120° by the firstdelay circuit 51 comprising, for example, a shift resistor, and thethird PWM dimming signal such that the second PWM dimming signal isdelayed by 120° by the second delay circuit 55, sets of fluorescentlamps made from the nine fluorescent lamps L1 to L9 (a set consisting ofL1, L4, and L7, a set consisting of L2, L5, and L8, and a set consistingof L3, L6, and L9) are sequentially operated with dimming. Consequently,light emitted from the backlight device 50 is operated as if the PWMdimming frequency were spatial averagely tripled. Accordingly,non-uniformity of luminance and color can be reduced to the same degreeas conventional ones, at a PWM dimming frequency which is aboutone-third of that conventionally required, and lighting efficiency isimproved.

For the liquid crystal panel 11, it is preferable to use either anOCB-mode liquid crystal panel or a TN-mode liquid crystal panel with acell gap of less than 5 μm (preferably about 2 μm), because in suchpanels the response time of liquid crystal is fast, and accordingly,edge blurring of a moving image can be further reduced.

In the present embodiment, the PWM dimming phases of the first andsecond PWM dimming signals are set to be shifted by 120° relative toeach other and the PWM dimming phases of the second and third PWMdimming signals are set to be shifted by 120° relative to each otherbut, needless to say, even if the phase difference is not exactly 120°,as long as is close to 120°, substantially the same effects areachieved.

As described above, according to the present embodiment, because lightsemitted from the fluorescent lamps L1 to L9, which are activated inresponse to the first, second, and third PWM dimming signals, arespatially averaged on the liquid crystal panel and the apparent PWMdimming frequency is tripled, colored interference fringes in a liquidcrystal display device resulting from the combination of the blackinsertion drive technique and the PWM dimming technique can be reducedto the same degree as conventional ones, at a PWM dimming frequencywhich is one-third of that conventionally required, and lightingefficiency can be improved compared to conventional ones.

INDUSTRIAL APPLICABILITY

As has been described above, according to the present invention, coloredinterference fringes in a liquid crystal display device resulting fromthe combination of the black insertion drive technique and the PWMdimming technique can be reduced, making it possible to displayhigher-quality images.

1. A liquid crystal display device for displaying images by irradiating a liquid crystal panel, which is driven in response to video signals, with light outputted from a backlight device, the liquid crystal display device comprising: drive means for driving the liquid crystal panel in response to the video signals such that one frame period is divided into a black display period and an image display period; a PWM dimming signal generation circuit for generating a PWM dimming signal for controlling the backlight device by a PWM dimming technique; a lighting circuit for driving the backlight device in response to the PWM dimming signal; and means for controlling a cycle and/or phase of the PWM dimming signal to prevent an occurrence of interference fringes in the liquid crystal panel, caused by the PWM dimming technique, wherein a PWM dimming frequency f (Hz) in the PWM technique and a ratio B (%) of the black display period to one frame period satisfy the relationship f≧25B+250.
 2. The liquid crystal display device according to claim 1, wherein the drive means is operable to control the PWM dimming signal generation circuit such that the PWM dimming frequency f (Hz) and the ratio B (%) satisfy the relationship f≧25B+250.
 3. A liquid crystal display device for displaying images by irradiating a liquid crystal panel, which is driven in response to video signals, with light outputted from a backlight device, the liquid crystal display device comprising: drive means for driving the liquid crystal panel in response to the video signals such that one frame period is divided into a black display period and an image display period; a PWM dimming signal generation circuit for generating a PWM dimming signal for controlling the backlight device by a PWM dimming technique; a lighting circuit for driving the backlight device in response to the PWM dimming signal; and means for controlling a cycle and/or phase of the PWM dimming signal to prevent an occurrence of interference fringes in the liquid crystal panel, caused by the PWM dimming technique, wherein: the backlight device is a direct-type backlight device having a structure in which a plurality of light sources are arranged in parallel and directly behind the liquid crystal panel; and the PWM dimming signal generation circuit is operable to dim all light sources of an order i in response to a first PWM dimming signal and dim all light sources of an order j in response to a second PWM dimming signal, the order i satisfying the relationship (2n−2)M+1≦i≦(2n−1)M and the order j satisfying the relationship (2n−1)M+1≦j≦2nM, wherein i and j (i, j=1, 2, 3, . . .) are natural numbers that represent the order of the light sources from one end of the backlight device, n (n=1, 2, 3, . . .) is an arbitrary natural number, and M (M=1, 2, 3, . . .) is an arbitrary natural number, the first and second PWM dimming signals being similar signals having phases that are shifted by about (PWM dimming cycle/2) relative to each other.
 4. The liquid crystal display device according to claim 3, further comprising a delay circuit for controlling the first and second PWM dimming signals such that the phases of the first and second PWM dimming signals are shifted by about (PWM dimming cycle/2) relative to each other.
 5. The liquid crystal display device according to claim 3, further comprising a control signal generation circuit for controlling the PWM dimming signal generation circuit such that the first and second PWM dimming signals synchronize to a synchronizing signal of an image.
 6. The liquid crystal display device according to claim 3, wherein the natural number M satisfies the relationship M=1.
 7. The liquid crystal display device according to claim 3, wherein the light sources arranged directly behind the liquid crystal panel are fluorescent lamps.
 8. A liquid crystal display device for displaying images by irradiating a liquid crystal panel, which is driven in response to video signals, with light outputted from a backlight device, the liquid crystal display device comprising: drive means for driving the liquid crystal panel in response to the video signals such that one frame period is divided into a black display period and an image display period; a PWM dimming signal generation circuit for generating a PWM dimming signal for controlling the backlight device by a PWM dimming technique; a lighting circuit for driving the backlight device in response to the PWM dimming signal; and means for controlling a cycle and/or phase of the PWM dimming signal to prevent an occurrence of interference fringes in the liquid crystal panel, caused by the PWM dimming technique, wherein: the backlight device is a direct-type backlight device having a structure in which a plurality of light sources are arranged in parallel and directly behind the liquid crystal panel; and the PWM dimming signal generation circuit is operable to dim all light sources of an order i′ in response to a first PWM dimming signal, dim all light sources of an order j′ in response to a second PWM dimming signal, and dim all light sources of an order k′ in response to a third PWM dimming signal, the order i′ satisfying the relationship (3n′−3)M′+1≦i′≦(3n′−2)M′,the order j′ satisfying the relationship (3n′−2) M′+1≦J′≦2(3n′−1)M′, and the order k′ satisfying the relationship (3n′−1)M′+1≦k′≦3n′M′, wherein i′, j′, and k′(i′, j′, k′=1, 2, 3, . . .) are natural numbers that represent the order of the light sources from one end of the backlight device, n′(n′+1, 2, 3, . . .) is an arbitrary natural number, and M′(M′=1, 2, 3,...) is an arbitrary natural number, the first, second, and third PWM dimming signals being similar signals in which phases of the first and second PWM dimming signals are shifted by about (PWM dimming cycle/3) relative to each other and the phase of the second PWM dimming signal and a phase of the third PWM dimming signal are shifted by about (PWM dimming cycle/3) relative to each other.
 9. The liquid crystal display device according to claim 8, further comprising a delay circuit for controlling the first, second, and third PWM dimming signals such that the phases of the first and second PWM dimming signals are shifted by about (PWM dimming cycle/3) relative to each other and the phases of the second and third PWM dimming signals are shifted by about (PWM dimming cycle/3) relative to each other.
 10. The liquid crystal display device according to claim 8, further comprising a control signal generation circuit for controlling the PWM dimming signal generation circuit such that the first, second, and third PWM dimming signals synchronize to a synchronizing signal of an image.
 11. The liquid crystal display device according to claim 8, wherein the natural number M′ is
 1. 12. The liquid crystal display device according to claim 8, wherein the light sources arranged directly behind the liquid crystal panel are fluorescent lamps. 